This application relates to radio communication systems and in particular to power amplifier linearization and up-conversion of digital signals to radio-frequency signals.
Mobile radio communication systems like those used for cellular telephone communication divide the radio-frequency (RF) portion of the electromagnetic spectrum into a multiplicity of individual signaling channels or frequency bands. Particular channels are allocated to particular users as they access the system. Each user's communication path is routed through the system through the channel allocated to that user. Signals transmitted in the system must be carefully regulated so that they remain within the channels allocated to the various users. “Out of band” signals can spill over from one channel to another, causing unacceptable interference with communications in the other channels.
In order to increase the data transfer rate supported by such RF channels, linear modulations of an RF carrier signal's amplitude like quadrature amplitude modulation (QAM), m-ary phase shift keying (PSK), etc. are utilized rather than the modulations typical of older communication systems, like constant-amplitude phase or frequency modulation (PM or FM). These new linear modulations require communication systems to use high-linearity power amplifiers and up-converters (modulators) to avoid causing interference in other channels in the system. These linearity requirements are especially difficult to meet when a single power amplifier is used to amplify more than one RF carrier signal. Such an amplifier is usually called a multi-carrier power amplifier (MCPA).
Conventional RF-power-amplifier linearization includes a feed-forward (FF) technique and variants that aim to improve the performance of the basic technique. The basic idea of feed-forward compensation of amplifier distortion was described by H. S. Black in 1928 in U.S. Pat. No. 1,686,792, and is depicted in FIG. 1. An input signal 101, shown in the frequency domain as two impulses (carriers), is divided into two parts by a splitter 102. One part of the input signal is provided to a main power amplifier (MPA) 103 that produces a distorted output signal 104, shown in the frequency domain as a plurality of impulses. A sample of the output signal 104 is derived from a signal sampling device, depicted in FIG. 1 as a coupler 105, and this sample is compared by a comparator 106 with the other part of the input signal that has passed through a delay element 107 and been delayed in time by an amount t1, that is equal to the temporal delay imposed by the MPA 103. The levels of the signals provided to the comparator 106 are adjusted such that the original input signal is cancelled out, leaving only the inverse of the distortion components from the MPA 103 as the difference signal produced by the comparator 105. This difference signal, referred to as an error signal hereinafter since it represents the error introduced by the MPA 103, is amplified by an error power amplifier (EPA) 108. The amplified error signal 109, shown in the frequency domain as a plurality of impulses, is combined by a coupler 110 with the rest of the output signal 104 that has passed through the coupler 105 and a delay element 111 and been delayed in time by an amount t2 that is equal to the temporal delay imposed by the EPA 108. The coupler 110 combines the signals from the two amplifier chains in such a way that the distortion is canceled, leaving only an amplified input signal as a combined output signal 112, shown in the frequency domain as a pair of carriers (impulses). The amount of performance improvement depends critically on accurate adjustments of the delay elements 107, 111, with respect to the MPA and EPA delays and of the signal levels provided to the comparator 106 and coupler 110.
The variants of the basic FF technique typically predistort the signal provided to the MPA to reduce the distortion imposed by the MPA before applying the correcting signals in the FF loop, thereby improving efficiency and reducing corrections needed in the FF loop. Examples of the FF technique and variants are described in U.S. Pat. No. 5,892,397 to Belcher et al. for “Adaptive Compensation of RF Amplifier Distortion by Injecting Predistortion Signal Derived from Respectively Different Functions of Input Signal Amplifier” and International Patent Publications WO 9923756 for “Cartesian Control Device for Telecommunication Systems” by Briffa et al.; WO 9945640 for “Feedback Controller Predistorter for Linearizing High Power Radio Frequency Amplifiers” by Kennington; and WO 9945638 for “Predistorter Circuit for High Power Radio Frequency Amplifiers” by Kennington. These documents show a general increase of complexity in the analog circuitry used in generating the predistortion signals for the MPA or use of only predistortion linearization without a FF loop for less demanding applications.
With semiconductor technology improving in both digital signal processor (DSP) realizations and analog-to-digital converter/digital-to-analog converter (ADC/DAC) realizations, efforts have been made to do the predistortion in the digital domain. Some efforts involved digital predistortion in linear modulation, single-carrier power amplifiers that are described in U.S. Pat. No. 4,291,277 to Davis et al. for “Adaptive Predistortion Technique for Linearizing a Power Amplifier for Digital Data Systems” and U.S. Pat. No. 5,049,832 to Cavers for “Amplifier Linearization by Adaptive Predistortion”, among others. Technical articles like James Carver, “Amplifier Linearization Using Digital Predistorter with Fast Adaption and Low Memory Requirements”, IEEE Trans. on Vehicular Technology vol. 39, no. 4 (November 1990) and Andrew S. Wright and Willem Durtler, “Experimental Performance of an Adaptive Digital Linearized Power Amplifier”, IEEE Trans. on Vehicular Technology vol. 41, no. 4 (November 1992) give insight into the evolution of digital predistortion techniques.
For the “multiple carrier” case, different approaches to digital predistortion and post-distortion are described in U.S. Pat. No. 5,732,333 to Cox et al. for “Linear Transmitter Using Predistortion”; U.S. Pat. No. 5,867,065 to Leyendecker for “Frequency Selective Predistortion in a Linear Transmitter”; U.S. Pat. Nos. 5,898,338 and 5,949,284 to Proctor et al. for “Adaptive Digital Predistortion Linearization and Feed-Forward Correction of RF Power Amplifier”; and U.S. Pat. No. 5,923,712 to Leyendecker et al. for “Method and Apparatus for Linear Transmission by Direct Inverse Modeling”.
FIG. 2 depicts the basic ideas disclosed in the patents to Proctor et al. and Leyendecker et al. One difference from Black's basic idea in FIG. 1 that will be seen is that the comparison implemented by the comparator 106 is done in the digital domain. In FIG. 2, a digital baseband (BB) signal generator 201 generates an input signal that is provided to a digital predistorter 202 and a digital FF correction module 203. An RF down-converter 204 receives a sample of the output signal produced by an MPA 205 produced by a coupler 206. An output signal produced by the down-converter 204 is provided to the correction module 203 that communicates with a digital correction estimator 207 that calculates tables to be used by the predistorter 202 for modifying the signal provided to the MPA 205 through a DAC and RF up-converter 208. The distortion imposed by the MPA 205 is thereby reduced. The remaining errors in the signal produced by the MPA 205 are compared in the digital domain in the digital FF correction module 203 that produces an error signal that is fed to a second DAC and up-converter 209 that drives an EPA 210. The EPA 210 cancels the remaining distortion at the output of the MPA by combining the error signal with the MPA's output signal in anti-phase through a coupler 211. Timing and gain adjustments are handled in the digital processing carried out by the correction estimator 207 and are used to update the real-time processing circuits 202, 203. The cited patents differ mainly in their implementations of the digital parts in devices 202, 203, 207.
An advantage of the implementation depicted in FIG. 2 is that all delay compensation in Black's original idea is performed in the digital domain. A disadvantage of this implementation is that the correction signal bandwidth is limited by the bandwidth of the digital correction circuits. The bandwidths of the DACs used in the up-converters 208, 209 and of the ADC in the down-converter 204 must be equal, and to correct third-, fifth-, or seventh-order distortion components, the bandwidths of the DACs and ADC must be at least three-, five-, or seven-times the bandwidth of the input signal. The bandwidths of currently available DACs and ADCs limit the bandwidth of the correction, making this implementation useful only in narrow-bandwidth applications.
The cited patents and publications rely on the fact that amplifier distortion can not be described only by measurements of two-tone intermodulation (IM) performance (e.g., third-, fifth-, or seventh-order IM performances) as described in the literature. The model for the amplifier must incorporate more data dimensions, taking account of so-called “memory effects”. By integrating the input signal over a predetermined time period, an estimate of the input signal's peak-to-average signal level is made. These estimates are used to create lookup tables of values that describe the device's performance dependence on factors other than the actual input signal strength. The well known two-tone IM measurement describes device performance only for a 3-dB peak-to-average swing of an input signal. If the input signal is a plurality of signals (e.g., a plurality of carriers), the peak-to-average values vary with time, yielding varying device performance.
U.S. Pat. No. 5,898,338 cited above describes one way to get peak-to-average signal estimates with a so-called “leaky integrator” and from those to create two-dimensional tables describing the performance of the amplifier. This patent and U.S. Pat. No. 5,949,283 and U.S. Pat. No. 5,959,500 to Garrido for “Model-Based Adaptive Feedforward Amplifier Linearizer” are different implementations of how to create look-up tables from observations of an amplifier's output signal and use the tables to pre-distort the amplifier's input signal to reduce the distortion of the output signal. These patents also deal with using the created look-up tables to create post-distortion signals that are subtracted from the main amplifier's output signal by another amplifier. U.S. Pat. No. 5,923,712 describes a method of reducing the size of these tables by using tables containing extracted finite-impulse response (FIR) filter coefficients.